The present invention relates to an improved process for acylating hydroxy-amino-organosulfonic acids, particularly hydroxy-amino-arylsulfonic acids.
The N-acyl-hydroxy-amino-arylsulfonic acids are wellknown intermediates in the preparation of water soluble dyestuffs. For example, the acylated "letter" acids, such as acetyl H-acid (8-acetylamino-1-naphthol-3,6-disulfonic acid) and benzoyl K-acid (8-benzoylamino-1-naphthol-3,5-disulfonic acid) are standard couplers for water soluble azo dyestuffs.
The traditional method for acylating hydroxy-amino-arylsulfonic acids to obtain the N-acyl derivative involves four procedural steps:
(1) dissolving the reactant in aqueous solution by neutralization with sodium hydroxide or sodium carbonate; PA0 (2) acylating with an anhydride or acyl halide at about neutral to moderately alkaline pH; PA0 (3) saponifying the acylated hydroxy group by alkalization and heating; and PA0 (4) isolating the N-acylated product by acidification of the reaction mixture to cause precipitation (salt added as needed to effect complete precipitation) followed by filtration.
This method has a number of disadvantages associated with it because of the oxygen acylation which competes with the desired nitrogen acylation. Excess acylating agent must be employed to drive the reaction to completion. The ester formed by acylation of the hydroxy group must be saponified which increases production time and consumes energy because of the heating required. The acid formed by saponification tends to show up in the final product. The product must be isolated by precipitation in most cases since the reaction solution contains too much salt to be practical as a dyestuff precursor. This precipitation increases production time and causes reduced yield of product since some product is lost to the mother liquor (75-85% yield is typical), and the mother liquor must be treated biologically since it contains product, salts and organic acid.